1. Field of the Invention
This invention relates in general to satellite communication networks and in particular to a method, apparatus and system for efficiently optimizing bandwidth and sharing resources between multiple users of a satellite communication network.
2. Discussion of the Background
Geo-synchronous satellite communication networks have existed for decades in various topologies and using various methods for sharing a fixed bandwidth channel between multiple users (Pritchard, Wilbur L., and Joseph A. Scivlli, Satellite Communication Systems Engineering, Prentice-Hall, 1986, incorporated in its entirely herein by reference). As these networks evolved, Time Division Multiple Access (TDMA) and Frequency Division Multiple Access (FDMA) bandwidth sharing techniques, and bandwidth efficient Quadrature Phase Shift Keying (QPSK) modulation have become de facto standards for Layer 1 (physical layer) although many other techniques and modulations are used to some extent.
Pure TDMA mesh networks exist, in which all user nodes take turns transmitting in a half duplex fashion. Pure FDMA point-to-point, or Single Channel Per Carrier (SCPC) networks, exist to allow full duplex transmission and reception. Many commercially viable network systems have evolved into a hybrid star topology, as shown in FIG. 1, taking advantage of the broadcast nature of geo-synchronous satellites by using a SCPC downstream carrier from a central hub to all user nodes, and using one or more TDMA upstream(s) shared by all user nodes to communicate with the hub.
FIG. 1 illustrates a conventional satellite communication network. A plurality of user nodes 12 communicate with a hub 11 via a satellite 10. The hub 11 transmits data to the user nodes 12 via broadcast downstream channels 13. Each of the user nodes 12 transmit data to the hub 11 via upstream channel 14.
A very popular and simple implementation of the TDMA upstream channel is to use an ALOHA technique enabling any user node to transmit to the hub any time the user node has data to send and relying on the probability that no other user node chose to transmit at that time, which would cause a collision. Once the demand for bandwidth exceeds 18.4% of the upstream channel (or 36.8% for slotted ALOHA), a more complex Layer 2 (media access control layer) methodology is required to schedule TDMA bursts so that collisions are prevented.
Many such Layer 2 methods exist, ranging from static configurations that permanently assign a given time slot to each user node, to more dynamic configurations which schedule bursts to service specific needs of individual user nodes.
As with any network system, the usefulness of control techniques at Layer 2 or higher is dependent on reliable performance of Layer 1. The geo-stationary satellite channel is well known to have an extremely long one-way delay (up to 300 ms) and it is extremely power limited due to the long propagation delay which results in the need to operate at the lowest signal-to-noise-ratio (SNR) possible in order to minimize the cost of radio-frequency (RF) equipment. Additionally, the channel is subject to further attenuation due to atmospheric conditions, some of which are variable, such as rain fade. If the SNR becomes too low, the bit error rate increases to an unacceptable level and all communication breaks down and creates adverse effects up the entire protocol stack. In particular, the Layer 2 slot allocation process breaks down very quickly for reliable protocol-protected data since the bandwidth demanded by user nodes is allocated by the system, but the user nodes' demand level is not reduced since data is lost and must be retransmitted. The situation worsens when the SNR is reduced so badly due to a fade condition that even station-keeping control information cannot get through and the user node actually drops out of the network. In this case the slot allocation process may waste valuable bandwidth before learning of the outage by allocating slots to the user node that the user node is unable to use.
Problems continue to arise as modern higher-level protocols are considered such as Voice over IP (VoIP) and streaming media which are sensitive to packet jitter. These protocols are designed to overcome the jitter of Internet Protocol (IP) traveling over terrestrial connections where the jitter is primarily due to queuing delays; however, the jitter introduced by a shared TDMA upstream satellite channel is often enough to degrade performance even when quality of the Layer 1 connection is excellent.
Any Layer 2 control protocol designed to maximize throughput and mitigate the performance problems experienced by users of satellite networks is itself subject to the same problems as the application data packets. Traditional control methods requiring handshaking are not very helpful since it takes too long, for example, to command one user node to stop transmitting on a TDMA slot, wait for an acknowledgement, and then command another user node to begin. Adding multiple upstream channels such that user nodes may transmit on any one of several channels helps matters somewhat since it increases the possible number of transmission opportunities per frame, however it does not solve the problem of the delay incurred while changes are made to the slot allocation.
An additional Layer 1 problem in traditional satellite networks, as recognized by the present inventors, is that the network's link budgets are over-designed due to the lack of adaptability of the modulation, demodulation, and FEC processes used therein. For example, if the threshold SNR of a link to achieve the minimum BER is 5 dB, a conventional link budget will include the amount of power required to achieve a 5 dB SNR plus XdB of rain margin (see Pritchard). The conventional approach is based upon the assumption that when it rains, there will be up to XdB of atmospheric attenuation so an SNR of (5+X)dB is established during clear sky conditions to ensure an SNR of at least 5 dB during heavy rain. During clear sky conditions, which occur most of the time for much of the world, XdB of link margin are wasted in the conventional approach.
FIG. 2 illustrates additional problems with conventional network communication methods, as recognized by the present inventors. This figure represents two timing scenarios, labeled as follows: No Synchronization, and Conventional Synchronization. In each scenario, HRxSOFn(ta) represents the nominal time a start of a data burst time frame (i.e. Start of Frame (SOF)) is received from user node “n,” and kmax*2 represents the maximum variation of reception time, or tracking error, for the worst case user node, due to minor perturbations in satellite position. The frame's relative frame sequence is “a.”
In the No Synchronization scenario, SOFs are unconstrained and arrive at various times due to variations between the propagation delays of each user node. In the Conventional Synchronization scenario, a time Rcdn (Remote Conventional Delay for User Node n) is added at each user node so that SOFs from all user nodes are received synchronously to the Hub's transmit reference SOF.
The conventional approach synchronizes HRxSOFn of each user node, but arrival time variation resulting from satellite tracking error disadvantageously results in some HRxSOFn occurring during hub frame n and some HRxSOFn occurring during hub frame n−1.
Further, in the conventional approach, Rcdn is chosen to synchronize HRxSOFn with subsequent Hub transmit SOF, which results in simultaneous arrival of SOFs sent at different times in the frame sequence. Thus, data bursts arriving at the Hub cannot be assumed to have all been sent during the same frame time, thereby complicating network control methods and increasing response time to control commands sent from the Hub.
As recognized by the present inventors, the industry is in need of a network system that combats these problems and enables standard off-the-shelf networking equipment using standard protocols to take advantage of a communication satellite's ability to reach user nodes at extreme distances and in areas of the world where broadband terrestrial communication is otherwise not practical.